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I have a volatile alkenyl bromide dissolved in hexanes. I have to use the halide in a Grignard reaction but separating it from hexanes is hard (the bromide is light sensitive and very volatile). I would like to use the solution of the halide in hexanes but I don't know how to determine its concentration. I tried taking an NMR of the solution but all the peaks interfere and I can't choose two characteristic peaks to take a ratio. Is there a titration technique or some other technique to determine its concentration?

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  • $\begingroup$ It'd be very helpful, if you are able to provide the structure of your alkenyl bromide. Also, how accurate it should be for the determination of the concentration? $\endgroup$ – Mathew Mahindaratne Aug 24 '20 at 20:30
  • $\begingroup$ For NMR you only really need one distinct peak of the alkyl bromide and one distinct peak of the internal standard. The latter can even be chosen so that its peak has a resonance frequency far away from your sample’s peaks. Is even that not possible due to spectral overlap? $\endgroup$ – orthocresol Aug 25 '20 at 1:13
  • $\begingroup$ Change the solvent to cyclohexane? $\endgroup$ – Karl Aug 25 '20 at 20:59
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You could form the Grignard and titrate it. The Grignard should be fairly straightforward to titrate (e.g. as described by Knochel). This also has the advantage of giving you precisely the concentration of your reactive specie. If you knew only the concentration of your alkenyl bromide, you would not know precisely the quantity of Grignard formed (as the reaction is not always perfect).

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The alkenyl bromide is likely allyl bromide, which has a boiling point of $\pu{71 ^\circ C}$, very close to that of mixed hexanes: $\pu{65-70 ^\circ C}$. Yes, those would be difficult to separate by distillation. Higher alkenyl bromides have boiling points higher enough to enable easier separation.

Two other possibilities present themselves, with reasonable accuracies possible, if you are moderately careful. First, allyl bromide has a density of 1.4697; hexane has a density of 0.6606. If you determine the density of the mixed product to $\pm 2\%$, you can determine the concentration to about the same accuracy - assuming linear concentration vs density, which is likely to be true, since hydrogen bonding or other special attractions do not seem evident.

A second method would be to determine the refractive index, which can be found to 4 decimal places - therefore perhaps more accurate than $\pm 2\%$. Allyl bromide has a refractive index of 1.4697 while hexane has a refractive index of 1.375. Refractive index of mixtures has been linear in my experience, tho I have no literature reference for that.

It is good practice to determine the concentration of your reactive ingredients by a method without side reactions. Unfortunately, running a test reaction to see how much Grignard reagent is formed may have an undetermined amount of the side reaction:

$$ \ce{CH_2=CHCH_2Br + CH_2=CHCH_2MgBr}$$ -> $$\ce{CH_2=CHCH_2CH_2CH=CH_2 + MgBr_2}$$

so it may not be easy to quantify what you are going to do next.

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